EP2685226A1

EP2685226A1 - Optical liquid level sensor

Info

Publication number: EP2685226A1
Application number: EP12400025.8A
Authority: EP
Inventors: Wolfgang Kreitmair-Steck; Frank Singer
Original assignee: Eurocopter Deutschland GmbH
Current assignee: Airbus Helicopters Deutschland GmbH
Priority date: 2012-07-11
Filing date: 2012-07-11
Publication date: 2014-01-15
Anticipated expiration: 2032-07-11
Also published as: US9116029B2; US20140014777A1; EP2685226B1

Abstract

The invention is related to an optical level sensor (1) for detection of liquid levels, said sensor (1) being mounted to a container or tank (9) comprising: a light source (2), a light detector (5) with a processing unit (6), and optical fibers (3), said fibres (3) comprising at least one input side (3.3) of the optical fibre, assigned to the light source (2) and reaching from the light source (2) into said container or tank (9), and at least one output side (3.4) of the optical fibre (3), assigned to the processing unit (6) via the light detector (5) and reaching from inside said container or tank (9) to the light source (2). A fibre collecting tube (12) is provided with support and guidance elements (4) which collect and guide all of the optical fibres (3, 3.3, 3.4) to different heights of the tank/container (9), each optical fiber (3) sensing a different liquid level.

Description

The invention is related to an optical liquid level sensor with the features of the preamble of claim 1.
Precise sensing of the level of liquids in tanks or containers is important for different reasons: prevention of overfilling of tanks, computation of the actual liquid consumption, identification of low liquid level, etc. From a safety point of view, it is even more important to know the exact level for tanks with inflammable liquids such as fuel. It is therefore necessary to control the filling and consuming processes as well as determining the current level if filling or consuming is to be started. Especially with inflammable liquids it is furthermore important for safety reasons to reduce or to eliminate all kind of electricity from inside the tank or container. It is important to avoid any kind of spark or ignition source inside the area/space of the liquid. Furthermore, in cases of liquid level sensors in a tank/container for aircrafts or vehicles it is important to design the liquid level sensor in such a way, that the components satisfy the crash requirements and that the liquid level sensor in case of a crash does not cause any damage to the tank/container, even if the tank/container is made of soft material.
The document WO 2010/051806 A1 discloses an optical sensor arrangement for detecting a first liquid medium in a second liquid medium by means of reflection of an emitted light beam at a wavelength, with a light source and an associated receiver, wherein two round glass rod lenses which are encapsulated in a housing are arranged parallel to one another. The glass rod lenses have an optical refractive index which is different from that of the liquid media. A reflection surface which is connected to the housing is arranged opposite the glass rod lenses. There is also a control device having a beam splitter, a second receiver and a third receiver which are arranged opposite one another. This sensor is suited only for measuring whether the liquid has reached a certain predefined level or not, but it is not suited for continuous measurements of varying levels. This sensor is therefore especially useful as a low level or maximum level sensor, but it cannot provide information e.g. for computing the consumption.
The document US 4 134 022 A discloses a level sensing apparatus having a source for supplying a signal having a predetermined frequency, a level sensor connected to the source and having an output for supplying an output signal having the predetermined frequency as long as the material, the level of which is being sensed, is not at a predetermined level. The level sensing apparatus further has a frequency sensitive circuit for receiving the output signal from the level sensor and for providing an output whenever the frequency of the signal is above or below the predetermined frequency, and a load which is connected to be responsive to the output from the frequency sensitive circuit. The level sensing apparatus is only capable to determine whether the liquid has reached a predefined level or not. Even the extension of the basic concept to several sensors is limited in its use, because it can only determine whether the liquid in one of the tanks has reached the respectively defined level or not. It is not possible to determine the exact level of the liquid.
The document US 3 535 933 A discloses a liquid level indicator in which a plurality of depending parallel light transmitting rods having different lengths are adjacently positioned, but maintained physically separated substantially throughout the entire longitudinal extent thereof. The upper portions of each of the light rods are interconnected and spaced apart by a flange section which in turn is clamped to the liquid container. The lower portions of the rods are integrally connected by a separator element between the rods and the upper portions provided an exposed liquid level indicating surface. The lower extremities of the light rods are positioned in the liquid and define a conical tip having an included angle of 90° for receiving light rays from and reflecting light rays to the upper indicating surface. The liquid level indicator needs a plurality of elongated members or rods and each of them needs a respective sensing unit. In order to provide the required stability, these members or rods need to be massive and have therefore significant weight. In addition, the light source and the light measuring device need to be positioned on the same end of the members or rods.
The document EP 1 533 598 A1 discloses a fiber optic level measurement device for use with transparent liquids in containers. Said fiber optic level measurement device has at least one optical fiber that is at least partially immersed in the liquid with the liquid level determined from the difference between injected light intensity and exiting light intensity. An input leg of the optical fiber has a spiral shape which extends to the lowest level of the container, while the output leg of the fiber is straight. Only one fiber measures the level of liquid based on the amount of refraction in the liquid. Therefore the measurement and the comparison of the light intensities of the input light and the output light needs to be very precise in order to determine the liquid level. Respective devices for measuring the light intensity need to be calibrated very precisely and are fairly expensive. Furthermore, the resolution of the sensor is limited due to the measuring principle.
The document DE 3 235 591 A1 discloses a fibre-optic liquid-level display device with a first optical fibre, assigned to a light source, and a second optical fibre, assigned to a light detector. The two optical fibres are coupled via a fibre coupler to a third optical fibre and a fourth optical fibre. The third optical fibre and the fourth optical fibre reach with their ends to different depths into the vessel containing the liquid to be measured. Depending on the level, the light at the ends of the optical fibres is coupled out or reflected and fed to the photodiode, with the result that the level range of the liquid can be detected by means of the current of said photodiode. The teaching of DE 3 235 591 A1 allows a reduction of the number of components needed, but it increases the requirements for the photo diode significantly, since it needs to discriminate the exact light intensity. The more different levels are to be detected, the more difficult is the realization of this sensor. The teaching of DE 3 235 591 A1 is therefore limited with respect to the achievable resolution.
Capacitive sensors as proposed in DE 102008064019 A1 , DE 102008042254 A1 or DE 102009029099 A1 suffer from the disadvantage that electricity has to be transmitted into the container/tank which is not desirable, especially for sensing the level of inflammable or explosive liquids. In addition, most capacitive probes contain metal probes and therefore tend to be in conflict with the crash requirements.
The object of the invention is to provide an optical liquid level sensor for determination of the exact level of a liquid in a container or tank, especially an optical liquid level sensor for a container or tank in a vehicle, such as an aircraft.
The solution is provided with an optical liquid level sensor according to the features of claim 1 of the present invention.
According to the invention an optical level sensor for detection of liquid levels is mounted to a container or tank. The optical level sensor comprises a light source, a light detector with a processing unit and at least one optical fibre with one input side of it assigned to the light source and reaching from the light source into said container or tank, and at least one output side assigned to the light detector and reaching from inside said container or tank to the light detector. A tube is provided with support and guidance elements which collect and guide all of the at least one optical fibres into or out of the tank/container. Preferably 50, 100, 200, 1000 or any number of optical fibres in between are provided in order to allow for detection of 50, 100, 200 or 1000 liquid levels. Each optical fibre is very small in dimension with typical diameters between 0.1 mm and 1.0 mm and embedded into a separate support and guidance elements made of plastic or metal. Each optical fibre is linked to detection of a specific liquid level.
The sensing of the light detector is such that if the light intensity of the light input communicated by the output optical fibre to the light detector is below a certain threshold l_c, the processing unit identifies the light to be off, i.e. the environment of the support and guidance element with the respective fibre is surrounded by liquid. If the light intensity is above the critical level l_c, the processing unit interprets it as surrounded by air, i.e. outside the liquid. Depending on the exact optical characteristics, especially the refractive index of the liquid, the diameter and the material of the fibre as well as the number of small turns of the fibre, such as 1 - 20 turns with radii < 10 mm, are defined in a way that due to the specific refractive indices of the fiber material and the liquid the light remains inside the respective fibre, if there is no liquid at the location of the corresponding support and guidance element, whereas enough light is leaving the fibre and entering the liquid, if the fibre is exposed to the liquid at the location of the corresponding support and guidance element. Advantages of the invention are:

● An optical liquid level sensor according to the present invention allows for a continuous sensing of the liquid level in a container/tank with a very high resolution without introducing electricity into the container/tank with the liquid,
● Despite the high resolution, the light source and the sensing electronics are very compact and simple,
● The inventive sensor is based on modular components and easy to assemble,
● Due to the possibility to use plastics or plastics skeletons for the sensor tube, it can easily be made crash worthy/crash tolerant, and
● Because of the multitude of sensor elements it is possible to provide for self-monitoring of the inventive sensor, e.g. by applying plausibility checks for the results of consecutive sensor elements.

According to a preferred embodiment of the invention the input sides and the output sides of the optical fibres are collected in groups respectively.
According to a further preferred embodiment of the invention the collection of input sides/ends of the fibres is fed collectively by a homogeneous planar light source.
According to a further preferred embodiment of the invention the collection of output sides of the fibres are sensed collectively by a Charged Coupled Device (CCD) as light detector.
According to a further preferred embodiment of the invention the optical fibres are made of glass or plastic.
According to a further preferred embodiment of the invention the optical fibres have a diameter between 0.1 mm and 1.0 mm.
According to a further preferred embodiment of the invention the support and guidance elements are made of plastic or metal.
According to a further preferred embodiment of the invention 50, 100, 200, 1000 or more sensor head elements are assembled.
According to a further preferred embodiment of the invention each optical fibre is guided in its half-length through its support and guidance element which is bending the fibre with a small radius < 10 mm around one or several guideways of said guidance element. The optical fibres are designed to reflect all of the light input or almost all of the light input inside the optical fibre to conduct said light input to the output end of the optical fibre, when the bent parts are in contact with air, whereas a large portion of the light is emitted to the liquid, when the bent parts of the optical fibres are in contact with the liquid.
According to a further preferred embodiment of the invention a reference optical fibre is used to provide a reference for the light intensity from an optical fibre which is exclusively in air.
According to a further preferred embodiment of the invention spacing elements and a spiral spring are provided, the support and guidance elements and spacing elements being compressed with the spiral spring in order to guarantee the density of the elements.
According to a further preferred embodiment of the invention the optical fibres can be arranged outside the fibre collecting tube with the optical fibres being more or less unprotected.
According to a further preferred embodiment of the invention the support and guidance elements including the optical fibres can be completely inside a caging basket.
According to a further preferred embodiment of the invention the transparent input window is an optical filter in order to reduce the light intensity or to increase the light discrimination level.
Preferred embodiments of the invention are presented in the following description with reference to the attached drawings.

Figure 1 shows a cross sectional view of an optical liquid level sensor according to the invention,
Figure 2 shows a spherical view of an extract of Fig. 1,
Figure 3 shows a preferred interpretation function of the optical liquid level sensor according to the invention,
Figure 4 shows a schematic view of an operation of an extract of Fig. 2,
Figure 5a, 5b show schematic views of operational modes of different optical fibers of the optical liquid level sensor according to the invention,
Figures 6 to 10 show schematic views for the support and guidance element of the optical liquid level sensor according to the invention.

According to Figure 1 an optical liquid level sensor 1 is mounted by means of screws or rivets 11 to a ceiling of a liquid container or tank 9 containing a liquid up to a level 19. An interface between tank/container 9 and the optical liquid level sensor 1 is sealed by a sealing element 9.1, made of e. g. elastomer material.
A fibre collecting sensor tube 12 for a multitude of polymeric or glass fibers 3 with respective diameters d_F which are guided at their respective measurement levels through the support and guidance elements 4 is mounted from inside the tank/container 9 by means of screws 10 to a housing 8.1. The support and guidance elements 4 are made of plastic or metal. The support and guidance elements 4 are flat with a height between 0.5 to 3 mm, allowing for a resolution of the liquid level 19 between 0.5 to 3 mm respectively.
If the tank/container 9 for a liquid is 500 mm high and the required resolution of the measurement is 1.5 mm there is a need of 333 fibres 3; the height of each of the support and guidance elements 4 might then be at most 1.5 mm and the diameter of the optical fibre 3 might be 1 mm or less.
Each of the support and guidance elements 4 encompasses one of the fibers 3. Spacing means 18 between said support and guidance elements 4 provide for an intended sensing scale of the liquid level sensor 1. A holding means 16 is provided inside the sensor tube 4 for bundling and guiding the fibers 3 from the support and guidance elements 4 to the interface with the sensor electronics 8. The support and guidance elements 4 guide each of the fibres 3 through one or up to twenty turns at a certain height/level in the tank/container 9. The collection of support and guidance elements 4 and spacing elements 18 are compressed by means of a spiral spring 13.
A sensor electronics 8 is mounted to the housing 8.1. This sensor electronics 8 comprises a processing unit 6, supplied by a combined power and signal line 7. Light is transmitted from a light source 2 inside the electronics housing 8.1 through a transparent output window 14. Another transparent input window 15 is integrated into the housing 8.1 for light reception at the CCD Unit 5 for sensing the received light pattern inside the electronics housing 8.1. The transparent input window 15 can act as an optical filter in order to reduce the light intensity or to increase the light discrimination level.
The individual fibers 3 are fed with homogeneous light from a planar light source, i. e. with light of the same intensity and color for the whole area of the planar light source, as light input 2 of the sensor electronics 8. The light is travelling because of total reflection at fiber walls through the fibers 3 without much distortion, i. e. with less than 25% of loss of initial light intensity at the input side in the fiber until the support and guidance elements 4. During the way of the light along the liquid/non-liquid environment of the support and guidance element 4 the amount of reflection depends on the presence of liquid. The intensity of the return light sensed at the optical output with the light detector as CCD Unit 5 of the sensor electronics 8 is reduced, if the part of the fiber 3 which is guided in the support and guidance element 4 is exposed to the liquid.
A reference for the light intensity of the light input 2 at normal, unaffected transmission in the fibers 3 is provided by a reference optical fiber 17.
According to Figure 2 corresponding features are referred to with the references of Fig. 1. Light is fed from the light input 2 into input sides 3.3 of the fibers 3. The light returned at output sides 3.4 of the fibres 3 is sensed with the CCD Unit 5. With the light input 2 yielding a homogenous illumination of the whole transparent output window 14, a bundle of input sides 3.3 of the fibers 3 is supplied with light of the same intensity and color. The bundle of output sides 3.4 of the fibers 3 projects individual light intensities 3.1 or 3.2 to the transparent input window 15, i. e. spot 3.1 due to light intensities I ≥ I_c or spot 3.2 due to light intensities I < I_c, said light intensities I depending on the amount of refraction and loss during the travel of the input light through the fibers 3 at the respective support and guidance element 4. The light pattern resulting on the input window 15 is collectively sensed by the CCD Unit 5 and assessed by the processing unit 6.
The transparent output window 14 is provided with an input spot 17.1 for the reference optical fiber 17 and the transparent input window 15 is provided with a measurement spot 17.2 for said reference optical fiber 17 for calibration of light quality and light level.
According to Figure 3 corresponding features are referred to with the references of Fig. 1, 2. The light intensities of the output ends 3.4 of the fibers 3 are executed with an interpretation function in the processing unit 6. The assumption for said interpretation function is, that at a critical light intensity I_c a distinction is made between two conditions of light transmission: 0 for no transmission and 1 for transmission. A light intensity I below the critical light intensity I_c results in the condition 0 with the interpretation that the support and guidance element 4 with the optical fiber 3 is surrounded by liquid. For condition 1 the interpretation is that the support and guidance element 4 with the optical fiber 3 is outside the liquid inside the container/tank 9.
According to Figure 4 corresponding features are referred to with the references of Fig. 1-3. The light source 2 of the optical liquid level sensor 1 illuminates homogenously the transparent output window 14 supplying the input side 3.3 with diameter d_F of the optical fibre 3 with light. The light source 2, the CCD Unit 5 and the processing unit 6 are powered by power lines 7.1. Signals from the output side 3.4 with diameter d_F of the optical fibre 3 pass through the transparent input window 15, are sensed by the CCD unit 5 and are transmitted via the signal line 7.2 to the processing unit 6 for interpretation. The information regarding liquid level interpreted by the processing unit 6 is communicated via an information line 7.3.
According to Figure 5a corresponding features are referred to with the references of Fig. 1-4. Light 3.5 is transmitted through the input side 3.3 of the optical fibre 3 towards a sensor head of optical fibre 3. The sensor head forms a turn with a radius r. The sensor head is in air/gas. The sensor head is designed by an appropriate selection of material for the fiber 3, the frequency or frequency spectrum of the light, and the turn radius r in such a way, that the light is to a very high amount kept inside the fiber 3 because of total reflection 3.6 of the light inside the fiber 3 if the fiber 3 is outside the liquid. The radius r is reduced to a minimum r_min, still returning a high throughput of light 3.8 through the output side 3.4 of the fiber 3 for still enough intensity I above the critical intensity I_c at the output side 3.4 of fibre 3 to the transparent input window 15.
According to Figure 5b corresponding features are referred to with the references of Fig. 1-5a. If the fiber 3 is submerged in the liquid below the liquid level 19 of the tank/container 9, the light 3.5 inside the fibre 3 is subject to a high degree of refraction 3.7 due to the physical behavior of the optical fiber sensor head. The refraction 3.7 results in a loss of light intensity I from the fiber 3 into the liquid. The reduced light intensity I in the return path 3.8 inside the output side 3.4 of the fiber 3 is lower than the critical intensity I_c.
According to Figure 6 corresponding features are referred to with the references of Fig. 1-5b. A support and guidance element 4 comprises a profiled lower section 4.1 with guideways 4.3 and an upper section 4.2 to be fixed onto the lower section 4.1 after insertion of the fiber 3 along the guideways 4.3. Figure 6a shows the lower section 4.1 of the support and guidance element 4 with four guideways 4.3 for the fiber 3 which allow for a turn radius r of the sensor head. Figure 6b shows the corresponding upper section 4.2 of the support and guidance element 4 in top view. The upper section 4.2 can be glued or fixed with other well-known techniques to the lower section 4.1 after insertion of the fiber 3 with its respective input side 3.3 of the fiber 3 and output side 3.4 of the fiber 3 along the four guideways 4.3 as shown in Figure 6c. Figure 6d shows a top view of the fiber 3 with the sensor head resulting from insertion of the fiber 3 into the support and guidance element 4. Figure 6e is a lateral view of the support and guidance element 4 with inserted fiber 3, the fixed section 4.2 on top of section 4.1 and the guideways 4.3.
According to Figure 7 corresponding features are referred to with the references of Fig. 1-6. An alternative support and guidance element 4 comprises the lower section 4.1 with modified guideways 4.3 for several turns to form the sensor head of the fiber 3.
According to Figure 8 corresponding features are referred to with the references of Fig. 1-7. The support and guidance element 4.1 with the inserted fiber 3 is mounted into a radial aperture of the sensor tube 12 with the sensor head of fiber 3 radial outside this aperture and the input side 3.3 of the fiber 3 and the output side 3.4 of the fiber 3 radial inside the sensor tube 12. The upper part 4.2 of the support and guidance element 4 has not yet been fixed to the lower part 4.1.
According to Figure 9a corresponding features are referred to with the references of Fig. 1-8. A lower part 4.1 of a further alternative support and guidance element 4 is provided with seventeen turns formed as bending sections around eight inner plus nine outer guideways 4.3 plus two guideways 4.3 at the interface of the further alternative support and guidance element 4 with the radial aperture of the sensor tube 12 for guidance of the input side 3.3 of the fiber 3 and the output side 3.4 of the fiber 3 radial inside the sensor tube 12. The further alternative support and guidance element 4 is provided with seven radial spacers 4.4 for positioning of the further alternative support and guidance element 4 relative to the sensor tube 12. Figure 9b shows the corresponding upper part 4.2 to be glued or fixed to the lower part 4.1 after mounting of the fiber 3.
According to Figure 10a corresponding features are referred to with the references of Fig. 1-9. The lower part 4.1 of the support and guidance element 4 with an already mounted optical fiber 3 with input side 3.3 and output side 3.4, respectively are completely mounted inside a sensor tube 12.1 executed as a screen basket in order to allow the liquid to flow easily into and out of the sensor tube 12.1. Radial spacers 4.4 regularly radial distributed at least on three positions of each support and guidance element 4 provide for centering of the respective support and guidance elements 4 inside the screen basket 12.1. Figure 10b shows the corresponding upper part 4.2 with radial spacers 4.4 to be glued or fixed to the lower part 4.1 into which the fiber 3 has been mounted.

Reference List

1: Optical Liquid Level Sensor
2: Homogeneous Planar Light Source/Light Feed by Pillar of Lamps
3: Polymeric or Glass Optical Fiber with diameter d_F
3.1: Spot on Transparent Window 15 with Light Intensity I >= I_c
3.2: Spot on Transparent Window 15 with Light Intensity I < l_c
3.3: Optical Fiber Segment for Injecting Light
3.4: Optical Fiber Segment for Returning Light
3.5: Injected Light Ray
3.6: Point of Total Reflection inside the Optical Fiber
3.7: Point of Partial Reflection and Partial Refraction
3.8: Reverberatory Light Ray
4: Support and Guidance Element made of Metal or Plastics
4.1: Lower Part of the Support and Guidance Element
4.2: Upper Part of the Support and Guidance Element
4.3: Guideways for the Optical Fiber
4.4: Radial Spacer
5: CCD Unit for Sensing the Received Light Pattern
6: Processing Unit
7: Combined Power and Signal Line
7.1: Power Line
7.2: Signal Line
7.3: Information Line
8: Sensor Electronics (including Light Source and CCD Unit)
8.1: Sealed Housing for the Sensor Electronics
9: Ceiling of the Liquid Container
9.1: Sealing Element for the Mounting of the Complete Sensor Unit onto the Liquid Container 9
10: Screws or Rivet for Mounting the Electronics Housing 8 to the Sensor Tube 12 with the Lower Sensor Elements
11: Screw for Mounting the Complete Sensor Unit onto the Liquid Container 9
12: Sensor Tube
12.1: Screen Basket as Sensor Tube
13: Helical Spring for Tightening the Support and Guidance Elements 4 and the Spacing Means 18
14: Sealed Transparent output window of the Housing of the Sensor Electronics
15: Sealed Transparent input window of the Housing of the Sensor Electronics, eventually realized as an Optical Filter
16: Holding Means/Mounting System for the Optical Fibers 3
17: Optical Fiber for Light Quality and Light Level Reference
17.1: Input Spot for the Light Quality and Light Level Reference
17.2: Measurement Spot for the Light Quality and Light Level Reference
18: Spacing Means to Guarantee the Intended Sensing Scale of the Liquid Level Sensor
19: Liquid Level to be Sensed

Claims

An optical level sensor (1) for detection of liquid levels inside a container or tank (9), said sensor (1) being mounted to said container or tank (9), said sensor (1) comprising:
a light source (2),

a light detector (5) with a processing unit (6), and

optical fibers (3, 17), said optical fibres (3, 17) comprising each

at least one input side (3.3), assigned to the light source (2) and reaching from the light source (2) into said container or tank (9), and

at least one output side (3.4), assigned to the processing unit (6) via the light detector (5) and reaching from inside said container or tank (9) to the light detector (2), characterized in that a sensor tube (12, 12.1) is provided with support and guidance elements (4) which collect and guide all of the optical fibres (3, 3.3, 3.4) to different heights of the tank/container (9), each optical fiber (3) sensing a different liquid level (19).
The sensor (1) according to claim 1,
characterized in that 50, 100, 1000 or any number of optical fibres (3) in between are provided.
The sensor (1) according to claim 2,
characterized in that the input side (3.3) of the optical fibres (3) and the output sides (3.4) of the optical fibres (3) are collected in groups respectively.
The sensor (1) according to claim 2,
characterized in that the collection of input sides (3.3) of the optical fibres (3) is fed collectively by a homogeneous planar light source (2).
The sensor (1) according to claim 2,
characterized in that the collection of output sides (3.4) of the optical fibres (3) are sensed collectively by a Charged Coupled Device (CCD) as light detector (5).
The sensor (1) according to claim 1,
characterized in that the optical fibres (3) are made of glass or plastic.
The sensor (1) according to claim 1,
characterized in that the optical fibres (3) have a diameter between 0.1 mm and 1.0 mm.
The sensor (1) according to claim 1,
characterized in that the support and guidance elements (4) are made of plastic or metal.
The sensor (1) according to claim 2,
characterized in that 50, 100 or up to 1000 sensor head elements are assembled in order to allow for detection of 50, 100 or up to 1000 different liquid levels (19).
The sensor (1) according to claim 1,
characterized in that each optical fibre (3) is guided in its half-length through its support and guidance element (4) which is bending the fibre (3) with a small radius r around one or several guideways (4.3).
The sensor (1) according to claim 1,
characterized in that a reference fibre (17) is provided as a reference for the light intensity from a fibre (3) which is in air exclusively.
The sensor (1) according to claim 2,
characterized in that spacing elements (18) and a spiral spring (13) are provided, the support and guidance elements (4) and spacing elements (18) being compressed with the spiral spring (13).
The sensor (1) according to claim 2,
characterized in that the optical fibres (3) are arranged outside the fibre collecting sensor tube (12).
The sensor (1) according to claim 2,
characterized in that the support and guidance elements (4) including the fibres (3) are completely inside a caging basket (12.1) as sensor tube (12).
The sensor (1) according to claim 2,
characterized in that the transparent input window (15) is an optical filter.